Diffusion pump



Sept 3, 1963 L. L. LEVENSON 3,102,678

DIFFUSION PUMP Filed March 8, 1961 2 Sheets-Sheet 1 INVENTOR. LEONARD L. LEVENSON AT TORNE Y Sept. 3, 1963 L. L.- LEVENSON 3,102,578

DIFFUSION PUMP Filed March 8. 1961 2 Sheets-Sheet 2 INVENTOR. L EONARD L.LEVEN$0N ATTORNEY United States Patent O 3,102,678 DEFFUSIGIN PUMP Leonard Lionel Levenson, Liverrnore, Calih, assi nor to the United States of America as represented by the United States Atomic Energy Commission Filed Mar. 8, 1961, Sen. No. 94,408

3 Claims. (til. 250-101) The present invention relates to diffusion pumps and, more particularly, to improvements in high vacuum diffusion pumps employing a cylindrically housed multistage nozzle system.

Conventional diffusion pumps known in the art are generally comprised of a multi-stage nozzle system, a cylindrical housing containing the nozzle system and, oftentimes, baffling material to prevent back diffusion. The normal manner of operating diffusion pumps of the type known in the art is to create jets of liquid (liquids most commonly used being oil or mercury vapor) which strike the Wall of the housing and collect in a pool at the lowermost portion of the diffusion pump. In this process molecules above the jet streams aretrapped in the jet streams and carried out of the pump, thereby evacuating the chamber above the pump. Numerous modifications as to nozzle geometries, bathe geometries, and heating systems for diffusion pumps exist in the art. One feature, however, generally common to all diffusion pumps known in the art is that the housing is a simple right cylinder having a flange or similar connection means at its upper 'end. Coupled with the fact that diffusion pump housings as known in the art are generally right cylinders is the fact that it is the general practice to position the housing and the nozzle system in relation to each other such that the housing wall extends above, as well as below, the point of contact of the uppermost jet stream and the housing wall. Thus, the area upon which the uppermost jet stream acts to evacuate the evacuation chamber is tical limits of size, diffusion pumps having diameters greater than 32 inches or 48 inches are rarely used. When such diffusion pumps fail to furnish the necessary pumping requirements, pumps having highly efficient nozzle systems and bathing systems must be used, and if they fail to meet the requirements a larger pump must be used.

The present invention provides a diffusion pump of practical size which is capable of pumping characteristics superior to those of the largest pumps now employed. It further provides a diffusion pump housing of novel geometry which increases the pumping speed of a conventional pump by ten to twenty-five percent. The present invention is adaptable to all diffusion pumps known in the tart, regardless of the particular type of nozzle system utilized. The outstanding results are achieved by employing a housing which has an increased diameter immediately above the point of contact of the uppermost nozzle stream and the housing Wall.

Several structures for providing the uppermost jet stream with an extended diameter above the point of contact with the housing wall are possible and as taught herein one such structure provides a housing of two cylinders of different diameters (whereas cylindrical housings are the most common in the art, housings of other geometries are adaptable for use by the present invention). The small diameter cylinder'serves as the lower portion 3,3025%; Patented Sept. 3, 1963 p CC of the housing, while the larger diameter cylinder extends from the point of contact of the uppermost .jet stream and the lower housing wall to an evacuation chamber. That is, the small cylinder extends vertically only far enough to provide a contact surface for the uppermost jet stream and no further. An alternative structure that provides an extended housing diameter above the uppermost jet stream contact point makes use of the evacuation chamber itself. The evacuation chamber becomes the extended diameter section of the housing. This is done by connecting the small diameter section of the housing directly to the evacuation chamber such that the contact point of the uppermost jet stream and the housing wall is at the juncture between the housing and the evacuation chamber.

An example of the improvement to be expected in the present invention is seen by considering a National Research Corporation diffusion pump, H-23P. In conventional form, the pump is housed in a cylinder 32 inches in diameter and will pump deuterium gas at a speed of about 60,000 liters per second at all pressures below 1 l0 mm. Hg. By increasing the diameter of the cylindrical housing above the point of contact between the uppermost jet stream and the housing wall to 48 inches, the pumping speed is increased to about 75,000 liters per second.

In most conventional systems, the base pressure of the evacuated enclosure is determined, essentially, by the equilibrium between the real and virtual leaks into the system and the rate at which gas is pumped out of this system. However, high temperature plasma processes present additional complications to the normal problems in high vacuum pumping. For example, in a plasma system using injection of neutral gas as part of a process, the rate at which gas is injected is normally much greater than the real or virtual leaks. Moreover, when a plasma is present, the walls of the enclosure can be affected by the plasma radiation. This radiation energy can cause some material absorbed on the wall to be liberated in the form of gases. Furthermore, some of the wall material itself can contribute to the gas contamination if the radiation is sufiiciently energetic to liberate substrate material. In order that the desired degree of vacuum be maintained during the plasma heating process, it is necessary to have a high pumping speed to quickly remove the vacuum contaminating sources. The present invention is used to obtain high pumping speeds at pressures below 1X10 The above example illustrates how the present invention is utilized to provide a diffusion pump with the necessary pumping speed to compensate for sources of gas which are above and beyond those normally found in a vacuum process. Wherein a conventional dififusion pump might possessthe necessary ability to achieve the desired vacuum, it may very well not possess the necessary pumping speed to remove contaminating gases Within the time required.

Accordingly, it is an object of the present invention to provide an improved diffusion pump.

It is another object of the present invention to provide a diffusion pump having superior pumping speeds.

It is a further object of the present invention to maintain a high vacuum in a chamber wherein high temperature plasma experimentation is a source of neutral gas contamination.

Other objects and advantages of the present invention will be made clear by a consideration of the following detailed description when taken together with the draw- FIGURE 2 is a schematic view of a cross section through the longitudinal axis of an embodiment of the invention wherein the nozzle system extends into the chamber to be evacuated.

Referring to FIGURE 1, a cylindrical shell 11 contains a multistage nozzle system 12 therein. Nozzle system 12 is shown schematically and represents nozzle systems in general since the present invention does not rely on the various schemes of creating jet streams for its source of novelty. At its closed lower end, cylinder 11 contains a reservoir 13 for the working fluid. Pipeline 14 leads to a torepump (not shown) and is connected into cylinder 11 between the reservoir 13 and lowest nozzle stage 16 of system 12. Nozzle 17 of system 12 represents all nozzle stages lying between the lowermost stage 16 and uppermost stage 18. Nozzle stage 18 is seen to provide a jet stream 19 of working fluid which is directed downward to contact cylinder 11 at its inner wall 21. Gas molecules ahove stream 19 become trapped in the stream when they contact it and are then taken out of the system. An annular flange 22 having an inside diameter equal to the inside diameter ot cylinder 11 and an outside diameter equal to the outside diameter of cylinder 23 is alfixed at the upper end 24 of cylinder 11. Cylinder 23, of greater diameter than cylinder 11, is affixed at its lower end 26 to flange 22. Cylinder 1-1, flange 22, and cylinder 23 thereby create a single housing 27 having an extended diameter immediately above the point of contact of stream 19 and wall 21.

The upper end of cylinder 23 has an annular flange 28 integral therewith for connection to flange 29 of vacuum chamber 31. A series of baffles 32 in cylinder 23 provide a means for mitigating hack diffusion of molecules.

In operation, the random movement of the neutral gas molecules causes the gas to be exposed to the first vapor jet stage 18 of nozzle system 12. Upon contact with the vapor stream 19, many of the gas molecules are forced through the pump and out of the system. The overall pumping speed, S (volume per unit time), of a multistage diffusion pump is a function, primarily of two factors: The first is the conductance S through the vapor jet stages; and the second is the conductance F through the region immediately preceding and connecting the first stage to the evacuated chamber. In a conventional pump comprising a single cylinder, both S and F are fixed values determined primarily by the diameter of this cylinder. The conventional approach to increasing pumping speed is to increase the size of the whole pump, i.e., the diameter of the cylinder, the nozzles, etc, whereby hoth S and F are increased. The present invention increases the pumping speed by a more efficient means concerned only with changing the conductance F.

Appreciation of the present invention is obtained by considering the [factors involved in increasing either S or F. To increase S, it is fundamentally necessary to enlarge the exposed area of the vapor streams and, consequently, increase the size of all of the associated elements. F, however, is easily made larger as is evident from the following relationship:

the various physical limitations only.

Referring now to FIGURE 2, an embodiment of the present invention wherein cylinder 23 of FIGURE 1 is dispensed with is shown. As mentioned supra, the present invention is most effective by providing a chamber of 4 largest possible diameter immediately above the point of contact of stream 19 and inner wall 21 of cylinder 11. It is evident that the largest diameter than can existv above the point of contact of stream -19 and inner wall 21 of cylinder 11 islimited by the dimension of the vacuum chamber 31 which is perpendicular to the central axis of the diffusion pump. To take tfull advantage of this large diameter, flange 22 of cylinder 11 is connected di rectly to chamber 31 where an opening equal in size to the opening at the top of cylinder 11 is provided. Ohamber 31 thereby takes the place of cylinder 23 of FIGURE l. Stream 19 still contacts cylinder 11 at its uppermost end 24. This embodiment of the invention is only adaptable tor use where the extension of the upper portion of nozzle system '12 into the vacuum chamber does not interrfere with the operations being performed within the vacuum chamber. Where the intrusion of nozzle system 12 into the vacuum chamber 3 1 does not intenfere with the desired operation taking place therein, the most effective and advantageous use of the present invention is thereby acquired.

The present invention achieves an improved diffusion pump by providing a housing geometry which takes the most effective advantage of the physical phenomena involved in diffusion pumping. The invention is, therefore, capable of increasing the pumping speed of all diffusion pumps now known in the art, no matter how sophisticated or refined they may be. The greatest pumping speeds now available with diffusion pumps can thereby he increased even further by the modification taught by the present invention.

Whereas numerous variation and modifications within the spirit and scope of the invention exist, it is not in tended that the invention be limited to the two preferred embodiments shown and described other than is set out in the following claims.

What is claimed is: r Y 1. In a vacuum diffusion pump the combination com prising a vertically oriented housing of constant crosssection having an open upper end and a closed lower end, an enclosure of greater cross-sectional area than I said housing secured in right-angularly outwardly stepped relation to the open end thereof, and jet stream means for producing a downwardly directed annular jet stream of liquid, said jet stream means coaxially disposed with respect to said housing in inwardly spaced relation to the peripheral wall thereof, said jet stream means including a portion projecting into said enclosure and directing said jet stream to impinge on the peripheral wall of said housing adjacent the juncture thereof with said enclosure.

2. The combination of a vacuum vessel having a flat horizontal wall portion including a circular openinga housing having a closed lower end and an open upper end, said housing having a constant inner diameter equal to the diameter of said opening, said housing secured at its open end to said vessel in coaxial relation to said circular opening, and diffusion pump jet stream means for producing a plurality of downwardly directed vertically spaced superposed annular jet streams of liquid, said jet stream means coaxially disposed within said housing inwardly from the inner peripheral wall thereof and including an upper end portion projecting into said vessel through said circular opening to direct the uppermost one of said jet streams to impinge on the inner peripheral wall of said housing adjacent its upper end.

3. The combination of claim 1 wherein said jet stream means are means for producing a plurality o-f superposed downwardly directed annular jet streams of liquid.

References Cited in the file of this patent UNITED STATES PATENTS 2,404,022 Alexander ct al. July 16, 1946 

1. IN A VACUUM DIFFUSION PUMP THE COMBINATION COMPRISING A VERTICALLY ORIENTED HOUSING OF CONSTANT CROSSSECTION HAVING AN OPEN UPPER END AND A CLOSED LOWER END, AN ENCLOSURE OF GREATER CROSS-SECTIONAL AREA THAN SAID HOUSING SECURED IN RIGHT-ANGULARLY OUTWARDLY STEPPED RELATION TO THE OPEN END THEREOF, AND JET STREAM MEANS FOR PRODUCING A DOWNWARDLY DIRECTED ANNULAR JET STREAM OF LIQUID, SAID JET STREAM MEANS COAXIALLY DISPOSED WITH RESPECT TO SAID HOUSING IN INWARDLY SPACED RELATION TO THE PERIPHERAL WALL THEREOF, SAID JET STREAM MEANS INCLUDING A PORTION PROJECTING INTO SAID ENCLOSURE AND DIRECTING SAID JET STREAM TO IMPINGE ON THE PERIPHERAL WALL OF SAID HOUSING ADJACENT THE JUNCTURE THEREOF WITH SAID ENCLOSURE. 